The present invention relates to an unfired refractory structural member in the form of a plate, for use as an expendable lining of metallurgical vessels, including ladles for molten steel, and more particularly, tundishes used in continuous steel casting. The structural member is made of a mixture comprising magnesia sinter and possibly olivine, an inorganic binder, a pore-forming material and possibly other additives, and water, by shaping, setting and drying.
The invention also relates to the use of this structural member as an expendable lining of a metallurgical vessel, particularly a tundish used in continuous steel casting.
In many cases, metallurgical vessels for molten steel have a refractory lining consisting of an outer lining which clings to the outer metal jacket and is composed of refractory bricks or another refractory material, and an inner or expendable lining which must often be replaced. According to Examined West German Specification No. 26 40 207, in order to protect surfaces that are exposed to fused metal, slags or hot gases, strongly compressed ceramically bound, mosaic-type refractory surface elements are provided which, among other things, can also be employed for tundishes used in continuous steel casting. However, these surface elements have a high apparent density, and particularly because of the large number of joints formed, do not lead to a satisfactory increase in the durability of the vessel lining.
An expendable lining for tundishes is disclosed in Unexamined West German Application No. 22 59 553, and consists of a lining of plates from a refractory heat-insulating material containing a refractory filler, refractory fibers, and an organic binder, such as starch or formaldehyde resin. The plates enable the molten steel to be supplied without previously heating the vessel. The heating by the molten steel should lead to a carbonization of the organic components and to a sintering of the inorganic components, so that the plates, prior to wetting and infiltration, are protected by the molten metal, and an easy breakability is attained when the expendable lining is removed. However, in many cases, the high ignition loss caused by the organic components and the production of carbohydrates is not desired. Furthermore, as the temperature rises, the strength of the plates diminishes considerably and the sintering remains limited to an external narrow area of the plates so that, particularly in the case of a varying meniscus, the expendable lining can easily be damaged.
Unexamined West German Application No. 27 16 092 discloses an expendable lining in the form of plates whose inner surface abuts the permanent lining and which is composed of sand, quartz powder, sintering means, mineral wool, paper, an organic binder and an inorganic binder. However, these plates also have a high ignition loss and the development of their strength through sintering its unsatisfactory.
European Pat. No. 0042897 provides, as an expendable lining, a mixture consisting of 65 to 85 parts by weight of magnesia sinter and 10 to 30 parts by weight of a finely powdered material (hydrated inorganic material) such as light magnesia, sodium phosphate or aluminum hydroxide, the mixture being formed from a watery suspension. However, plates made from the mixture, with a large amount of the very finely powdered reactive material, do not possess a uniform strength or a long durability and, during preheating, a structure is produced that is full of cracks.
Furthermore, an article published in "Keramische Zeitschrift", 33, 1981, page 518, table 8, proposes the use of plates made of magnesium oxide as inner lining for tundishes used in continuous steel casting. However, these plates have a high ignition loss of 7 to 10% by weight.